Dominant effects of 2D pores on mechanical behaviors of plasma sprayed ceramic coatings during thermal exposure

Journal Pre-proof Dominant effects of 2D pores on mechanical behaviors of plasma sprayed thermal barrier coatings during thermal exposure Li-Shuang Wang, Chun-Hua Tang, Hui Dong, Guang-Rong Li, Guan-Jun Yang PII:

S0272-8842(19)33360-7

DOI:

https://doi.org/10.1016/j.ceramint.2019.11.168

Reference:

CERI 23537

To appear in:

Ceramics International

Received Date: 24 October 2019 Revised Date:

12 November 2019

Accepted Date: 19 November 2019

Please cite this article as: L.-S. Wang, C.-H. Tang, H. Dong, G.-R. Li, G.-J. Yang, Dominant effects of 2D pores on mechanical behaviors of plasma sprayed thermal barrier coatings during thermal exposure, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.168. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Dominant effects of 2D pores on mechanical behaviors of plasma sprayed thermal barrier coatings during thermal exposure

Li-Shuang Wang1, Chun-Hua Tang2, Hui Dong1, Guang-Rong Li2*, Guan-Jun Yang2 1. School of Materials Science and Engineering, Xi’an Shiyou University, Xi’an, 710065, China 2. State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, 710049, China

*: Corresponding author: Dr. Guang-Rong Li State Key Laboratory for Mechanical Behavior of Materials School of Materials Science and Engineering Xi’an Jiaotong University Xi’an, Shaanxi, 710049, P. R. China Tel.: ++86-29-82665299 Fax: ++86-29-83237910 E-mail: [email protected]

Abstract Realizing engineering application of new candidate ceramic materials is highly required for the development of advanced thermal barrier coatings (TBCs). In addition to the optimization of intrinsic properties of ceramic materials, a feasible way is to tailor the structure of coatings based on the understanding of structure-properties evolution during thermal exposure. Herein, the unique sintering behavior and the consequent effect on spallation of plasma sprayed coatings made of a candidate material, i.e., La2Zr2O7 (LZO), were investigated by experiments and simulation. Experimental results showed that significant changes occurred for the porosity, 2D pore density and hardness during thermal exposure. The 2D pores are the essential microstructural characteristics and are dominantly responsible for the changes of mechanical property. Simulation results suggested that the healing of 2D pores increase the driving force for crack extension. This is the main cause for the spallation of ceramic top coat during thermal cyclic test. Finally, some structural designs towards long lifetime of TBCs were discussed. The dominant effects of 2D pores on mechanical properties and failure of TBCs provide fundamental understanding to the structural tailoring of advanced TBCs for future applications.

Keywords: thermal barrier coatings, La2Zr2O7, 2D pores, mechanical properties, structural design

1. Introduction Thermal barrier coatings (TBCs) endow the underlying metallic components with desirable capacity to solidly work at high temperature. Thus, the TBC become an essential protective layer in various fields associated with extremely high temperatures, e.g., aircraft engines and land-based gas turbines [1-7]. The function of TBCs is to effectively prevent the heat flux and form a temperature drop across thickness. Given that, the materials selected for TBCs should have low thermal conductivity and high fracture toughness. The latter is even important, since the durability is the fundamental support for the thermal insulation. In past decades, yttria-stabilized zirconia (YSZ) has been found to be a desired material for the TBCs [8-12]. In addition to its low thermal conductivity (approximately 2.5 W·m-1·K-1 at room temperature for bulk YSZ [13]), YSZ exhibits a unique property of ferro-elasticity that benefits the toughness. Therefore, the YSZ is rare to be comprehensively responsible for the thermal insulation and lifetime. However, the development of engines and turbines requires a higher inlet temperature, which brings challenge for the YSZ. It is reported that the YSZ-prepared TBCs are more readily to fail when they work at higher temperatures (e.g., 1200 °C and above) [14]. The cause is that the non-equilibrium tetragonal phase changes to stable cubic phase and monoclinic phase, resulting in 3%~5% volumetric changes and decrease in fracture toughness [15, 16]. Therefore, candidate materials with stable phase are highly required for the advanced TBCs in future application [17-19]. As a new potential material, La2Zr2O7 (LZO) were widely investigated in recently years [20-22]. The thermal conductivity of LZO (1.56 W·m-1·K-1 at 1273 K for bulk LZO [20, 23]) is lower with respect to that of YSZ. Moreover, no phase change would occur until the melt point of LZO, i.e., 2573 K [24]. However, a challenge for the application of LZO is that it has a lower fracture toughness and thermal expansion coefficient (TEC) compared with YSZ. As a result, poor durability of LZO was observed

during thermal cyclic test [25, 26]. This opposite their potential to be candidates used for TBCs. To solve this problem, Feng et al. [27-29] investigate several new materials with a similar crystal structure with YSZ, which endows them with the unique ferro-elasticity. However, the related investigations were mainly based on bulk materials to deal with some intrinsic properties. The performances in real service are still needed to be tested. Another way to accelerate the engineering application of new materials is to tailor strain tolerant structures, with the aim to overcome the weak fracture toughness of bulk materials. The structure design often requires deep understanding on the structure change during thermal exposure. Some thermal cyclic tests revealed that the LZO-prepared TBCs often fail by a form of partial spallation [30]. In fact, the as-deposited LZO coatings present an acceptable strain tolerance, owing to their ＞50% drop of elastic modulus with respect to the bulk LZO [22]. However, sintering-induced stiffening inevitably occurs, which cause significant changes in mechanical properties [31]. The failure of LZO-prepared TBCs should be highly associated with the mechanical properties. Nevertheless, the underlying mechanism and the dominant effect are still not clear until now. The objective of this study is to correlate the sintering-induced changes in mechanical properties with microstructure. Firstly, the changes in mechanical property and microstructure were investigated during thermal exposure. Subsequently, the dominant structural change was correlated with the change in mechanical property. Finally, the effect of sintering on interfacial cracking was investigated by simulation, and the strain tolerant design to resist spallation was discussed. This would make fundamental contribution to the engineering application of new TBC materials in future applications. 2.

Experimental

2.1 Sample preparation and thermal exposure Commercially available LZO powders (12 to 85 µm, Yiyang, China) was plasma sprayed (GP-80, 80 kW class, Jiujiang, China) on stainless substrate. Total thickness of the LZO coating is approximately 300 µm. Table 1 shows parameters of plasma spraying used in this study. After deposition, the substrate was removed by hydrochloric acid. The thermal exposure was conducted in an isothermal furnace with a temperature of 1250 °C. The heating and cooling rates were set to be approximately 10 °C/min. Table 1 Parameters of the plasma spraying. Parameters

Value

Plasma arc power, kW

36

Flow rate of primary gas (Ar), L/min

55

Flow rate of secondary gas (H2), L/min

5.5

Flow rate of powder feeding gas (N2), L/min

6

Spray distance, mm

110

Torch traverse speed, mm/s

600

2.2 Microstructural characterization and determination of hardness Microstructural changes were mainly observed by scanning electron microscopy (SEM, TESCAN MIRA3, Brno, Czech Republic). Firstly, the apparent morphology in cross-section was observed, along with the apparent porosity. The apparent porosity was determined by image analysis with 1000× polished SEM images. Subsequently, a quasi in-situ method was used to observe the detailed healing behavior of 2D pores caused by sintering. The details of in-situ observation can be found elsewhere

[32], and would be briefly described herein as follows: (i) a target 2D pore is found in a sample at high magnification (e.g., 50000×); (ii) decrease the magnification to a low magnification to show the total morphology of the sample; (iii) position the target pore by coordinates; and (iv) after thermal exposure, reposition the target pore based on the coordinates. In addition, the change in 2D pores was also presented by collecting the length of 2D pores in unit area before and after thermal exposure. Commonly, the length was collected based on 20 polished SEM images at 5000× magnification. Vickers hardness was determined on polished cross-section of samples (Buehler Micromet 5104, USA). The load was set to be 300 gf with a loading duration of 30 s.

3. Finite element simulation Finite element simulation was used to investigate the sintering effect on interfacial cracking [33-35]. Figure 1 shows the developed model. The model consists of substrate (including bond coat) and top coat, so that mismatch strain can be applied on the top coat. In order to facilitate the simulation, Young’s modulus was used as the input data. The changing range of the Young’s modulus was obtained based on the equivalent normalized values (with respect to bulk LZO) compared with hardness. The hardness and Young’s modulus of bulk LZO is Hv190 and 175 GPa, respectively [22, 23, 36]. Stiffening degree is defined as the changing degree of Young’s modulus after thermal exposure with respect to the as-deposited state. Considering the fact that TBCs are exposed to gradient thermal exposure in real service, the cases of both even and gradient stiffening degrees were investigated, as shown in Fig. 1(a) and 1(b). The latter case was simplified to two layers with different Young’s modulus (called E1 and E2), and each layer has a thickness (h) of 150 µm. A pre-existing cracking is set at the middle of the top coat. The thickness (a) of substrate and bond coat is 3.1 mm, and the length (w)

is 8 mm. To simulate the TEC mismatch strain during service, a uniform strain of 0.3% was applied on the left boundary, while a symmetric constraint is introduced at the right boundary excluding the region above the pre-existing crack, as shown in Fig. 1(c). The driving force for extension of interfacial cracks is evaluated by strain energy release rate (SERR). Fine enough meshing size near the crack is set to avoid the effect of grid size. The SERR is determined based on the virtual crack closure technique (VCCT), as shown in Fig. 1(d) [37, 38]. The VCCT mainly deals with cases based on linear elastic fracture mechanics. In order to focus on the effect of stiffening degree, no crack extension occurs in this study. In a four-node element, the SERR components Gi (i=I, II) can be obtained using the nodal force at the crack tip and the nodal opening displacement behind the crack tip. The total SERR G is the sum of the SERR components, as shown in the following equations [39]: GΙ = GΙΙ =

Fy ∆ν 2B∆α

=

Fy (ν 3 −ν 4 )

(1)

2B∆α

Fx ∆µ Fx ( µ3 − µ4 ) = 2B∆α 2B∆α

G =GΙ + GΙΙ

(2)

(3)

where ∆α refers the crack growth increment, Fx and Fy correspond to the shearing and opening force, respectively, µi and νi are the shearing and opening displacement of the nodes behind the crack tip along x and y-axes, respectively, B is a constant in 2D model.

Fig. 1 Model development: (a) schematics for a case with even stiffening degree across the thickness of top coat, (b) schematics for a case with simplified gradient stiffening degree across the thickness of top coat, (c) geometry, strain and constraints applied on model, (d) schematic of the VCCT for four-node elements [38].

4. Results and discussion 4.1 Evolution of global structure Figure 2 shows fractured cross-section of LZO coating at as-deposited state. Distinct lamellar structure can be observed from the global view. This is a typical structure for the plasma sprayed ceramic coatings, which are formed by stacking solid splats in a layer-by-layer pattern [40, 41]. The thickness of each layer is approximately several microns. The coating is obvious a porous structure that is

required by the effective prevention of heat flux, because the thermal conductivity of air (0.025 W·m-1·K-1) [42] is significantly lower than that of the solid LZO (1.56 W·m-1·K-1) [20]. However, different from the conventional porous structure with only globular voids, the pores in LZO coating appear to be multiple orientations and morphologies. From the view of morphology, these pores can be divided into two types: 2-dimensional (2D) and 3-dimensional (3D) morphologies. The 3D pores are called globular voids, which is resulted from incomplete cover of molten droplets onto the rough surface [43]. From the view of orientation, the 2D pores further include two kinds: inter-splat pores that are perpendicular to heat flux, and intra-splat cracks that are parallel to heat flux. The large quantity of 2D pores make plasma sprayed ceramic coatings unique porous structures [40, 44]. The thermal and mechanical properties are dominantly determined by these 2D pores. As a result, the porosity of the plasma sprayed ceramic coatings is approximately 10%~20% [40], whereas the thermal and mechanical properties are less than 50% of the corresponding bulk materials [41, 45, 46]. In fact, the 2D pores often comprehensively affect the properties, despite of their orientations. For example, the inter-splat pores are often recognized as the main ones to prevent heat flux [22, 42]. However, the effect of intra-splat cracks cannot be neglected [47], even though their orientations are mostly parallel to heat flux. The cause is that the distribution of intra-splat cracks would significantly change the heat flux. Therefore, the 2D pore network, which means the inter-splat pores are often connected with the intra-splat cracks, is the intrinsic characteristic for the plasma sprayed ceramic coatings. The degradation of LZO coating during thermal exposure would be highly associated with the change of 2D pore network.

Fig. 2 Fractured cross-section of as-deposited LZO coatings.

Figure 3 shows the evolution of polished cross-section during thermal exposure. Three significant changes can be recognized. Firstly, initial highly porous structure changes to be much denser. From Fig. 3(a), some large voids with approximately several tens of microns can be observed. However, they disappear after thermal exposure. In fact, most of these large voids come from the preparation of polished samples [48, 49]. At the as-deposited state, some splats are easily to spall off due to their weak bonding. After thermal exposure, the bonding between splats is significantly enhanced. As a result, no large void can be observed from Fig. 3(d). Secondly, the quantity of 2D pores significantly decreases, despite of inter-splat pores and intra-splat cracks. Moreover, the disappearance of 2D pores is highly related with their widths. A smaller width is more readily to disappear. As a result, only some 2D pores with large width can be found in Fig. 3(d). This is consistent with previous report [50], which revealed the effect of pore width on its healing behavior during thermal exposure. Thirdly, the initial continuous 2D pores are firstly divided into several segments, and then these segments change to globular morphology. In brief, the main structural change of LZO coatings during thermal exposure is the healing of 2D pores. Moreover, the healing may proceed in a multiple contact pattern.

Fig. 3 Evolution of polished cross-section during thermal exposure: (a) as-deposited state, (b) 10 h, (c) 50 h, and (d) 200 h.

4.2 Relationship between microstructure and hardness This section aims to correlate the changes of microstructure and mechanical property, so as to figure out the dominant factor responsible for the degradation during thermal exposure. Figure 4 shows the changes of porosity, 2D pore density and hardness during thermal exposure. In order to facilitate comparison, all the values were normalized with respect to that at as-deposited states. The changing degree was obtained based on the follow equation:

yh =

xh − x0 x0

(4)

where yh is the absolute changing degree at h dwell time, xh is the value (porosity, 2D pore density and hardness) at h dwell time, x0 is the corresponding value (porosity, 2D pore density and hardness) at as-deposited states.

Some similar changing trends can be found between microstructure and hardness. First is that the changes are much faster at initial dwell time (e.g., less than 10 h). Second is that the initial dwell time completes most of the changing degrees in total thermal exposure, i.e., 83%, 66%, and 83% for porosity, 2D pore density and hardness, respectively. These suggested that the total sintering process can be approximately divided into two stages at least. Similar stage-sensitive evolution can be found in coatings made of yttria-stabilized zirconia [51]. However, the changing degrees shown in Fig. 4(b) also suggest that the hardness has a closer association with the 2D pore density than that with the porosity. During thermal exposure, their changing degrees are comparable, and most of the values are over 50%, which is significantly greater than that of the porosity. Figure 5 shows the relationship between hardness and microstructure. Fitting lines were obtained based on the existing data. The hardness was normalized to bulk LZO, whereas the porosity and 2D pore density were normalized to the corresponding as-deposited states. When the porosity and 2D pore density decrease to 0, the normalized hardness should be 1. The predicted limit values based on 2D pore density and porosity are 1.02 and 1.4, respectively. Therefore, it can be concluded that the hardening of LZO coating is mainly caused by the healing of 2D pores.

Fig. 4 Changes in microstructure and hardness during thermal exposure: (a) normalized values with respect to the as-deposited coatings and (b) changing degree.

Fig. 5 Relationship between hardness and (a) apparent porosity and (b) 2D pore density. The hardness was normalized to bulk LZO, whereas the porosity and 2D pore density were normalized to the corresponding as-deposited states.

4.3 Healing of 2D pores during thermal exposure Figure 6 shows the healing of 2D pores with different widths. After a same dwell time (5 h), the 2D pores with a smaller width (Fig. 6a) are almost totally healed, whereas the 2D pore with a larger width (Fig. 6b) remains open. This is consistent with the phenomena observed in Fig. 3. In fact, the healing of 2D pores proceeds in two different ways that associated with the roughening degree. After deposition, the solid splats are formed by columnar grains due to rapid growth after nucleation at cold contact interface [52]. The splat surface, which is actually the pore surface, is relatively smooth at as-deposited state. After thermal exposure, the initial smooth surface becomes roughening. The underlying cause is the matter transfer driven by decreasing the free energy of total system. It is reported that the roughening pattern can be grain boundary grooving and surface faceting [53, 54], because these regions have high free energy. The first healing way is that the roughening occurs in a narrow pore, and the counter-surfaces are contacted in multi-points. This bridge-connected behavior actually provides more ways for matter transfer. As a result, the sintering would be faster and the narrow gap would be healed

in short time. The second healing way is that the roughening occurs in a wide pore, and the counter-surfaces cannot be contacted. The matter transfer can only proceed through the existed bonded area. As a result, the sintering would be slower and the wide pore can exist for long time. Based on the discussion above, the width of 2D pores and the roughening degree are two critical parameters to determine the healing process. Liu et al. [22, 50] quantitatively characterized the roughening degree of LZO dependent on temperatures and proposed a critical width for 2D pores to retard healing. Based on this critical value, sintering-resistant TBCs can be achieved by tailoring the width of 2D pores [55].

Fig. 6 In-situ healing of 2D pores during thermal exposure: (a) a pore of less than 50 nm width, and (b) a pore of approximately 200 nm width.

4.4 Effect of microstructural changes on cracking behavior Sintering of ceramic top coat is a main factor to result in failure of TBCs. This is caused by the detrimental effect of 2D pore healing on the strain tolerance. Therefore, this section would discuss the healing of 2D pores on cracking behavior of LZO coatings.

4.4.1 Effect of crack length on the driving force of interfacial cracking The model developed in section 3 was used to investigate the interfacial cracking. Figure 7 shows the normalized SERR as a function of cracking length. It is found that the normalized SERR decreases from nucleation to initial extension of an interfacial crack. After certain extension, the normalized SERR tends to be unaffected by the crack length. This is consistent with other report [56]. In order to avoid the effect of crack length, the crack length was set to be 100 µm in the following sections.

Fig. 7 Effect of crack length on the normalized SERR of interfacial cracking.

4.4.2 Effect of microstructural changes on the driving force of interfacial cracking Figure 8 shows the effect of 2D pore healing on the driving force of interfacial cracking. The stress ahead of crack tip significantly increases during the sintering process. In addition, the driving force for interfacial cracking after thermal exposure is approximately 2 times with respect to the as-deposited state, as shown in Fig. 9. Figures 10 and 11 show the comparisons of stress and SERR between isothermal and gradient cyclic tests. Based on Figure 10, the stress concentration at crack tip is weaker for the case of gradient cyclic test. It is interesting that the total SERRs remains unchanged for the two cases. The reason is that the total SERR for a crack is mainly determined by the Young’s modulus and the thickness of the upper region, as shown in the following equation [57]:

Gi = ∫

h

0

Ex ε 2 dh 2(1 − υ )

(5)

where Gi is the SERR, h is the distance of crack to coating surface, Ex is the Young’s modulus, ε is the applied strain, ν is the Poisson’s ratio.

Fig. 8 Effect of microstructural changes on stress distribution at crack tips: (a) tensile stress; and (b) shear stress.

Fig. 9 Effect of microstructural changes on driving force (SERR) of interfacial cracks.

Fig. 10 Comparison of stress distribution between isothermal and gradient thermal cycling test: (a) tensile stress; and (b) shear stress.

Fig. 11 Effect of microstructural changes on driving force (SERR) of interfacial cracks.

However, the failure pattern of TBCs after isothermal and gradient cyclic tests are often different. The spalled thickness of top coat in isothermal cyclic test is often larger than that in gradient thermal cyclic test. For example, when spallation occurs, the residual thickness in isothermal cyclic test is less than 20% [58], whereas it is often larger than 50% in gradient cyclic test [59]. In fact, this is a result of the co-change of driving force and fracture toughness caused by sintering. The healing of 2D pores causes

the increase of mechanical properties, including Young’s modulus and fracture toughness. The former means the increase of driving force [57], while the latter refers to the increase of cracking resistance. Moreover, the increase degree of mechanical properties is positively correlated to the healing degree of 2D pores. At any case, spallation occurs when the driving force (i.e., SERR) is larger than the fracture toughness. Based on the Fig. 11, the driving force is comparable for these two cases. However, in the case of isothermal cyclic test, the top coat has the same fracture toughness across the thickness. In contrast, the fracture toughness should decrease at the lower region of the top coat, because it would be exposed to lower temperatures due to the gradient thermal condition. As a result, the crack would be more readily to extend. This can be qualitatively responsible for the smaller spallation thickness in gradient thermal cyclic test. It is worth noting that the gradient thermal condition is simplified in this study. For a quantitative investigation in future, it is necessary to model the real service conditions. In brief, the healing of 2D pores causes the changes in mechanical properties, which are main factors to result in failure of TBCs, as well as degradation of thermal insulation reported in previous study. The cracking is actually a result of the driving force exceeding the toughness. Therefore, the ways to resist cracking can be summarized through three aspects: (i) decreasing the driving force. A typical example for this way is the dense-vertically-cracked TBCs (DVC-TBCs) [60, 61]. The vertical cracks across the thickness of the DVC-TBCs enable them excellent strain tolerance even after thermal exposure. Consequently, the driving force remains in a relative low value and the lifetime of DVC-TBCs can be extended by several times; (ii) enhancing fracture toughness. This way can be widely found in the double-ceramic-layer design (or even multi-layers) [58]. The lower layer attached to bond coat often have a higher fracture toughness, which can be achieved by process optimization. The higher fracture toughness can enhance the cracking resistance, and thus extend the lifetime of TBCs; (iii) decreasing

the driving force and meanwhile enhancing fracture toughness. This is a challenge for the design of TBCs. Lv et al. [62] proposed a gradient porous structure and proved its better sintering resistance compared with conventional coatings. A further test still needs to be done for the lifetime evaluation. Cheng et al. [30, 63] designed a multiple-layered TBCs with a smaller total thickness based on equivalent thermal insulation. The lifetime of TBCs can be extended without sacrificing the thermal insulation.

Conclusions In this study, the changes in microstructure and mechanical property of plasma sprayed LZO coatings during thermal exposure were investigated. The dominant microstructural factor was correlated to the hardness, and the effect on spallation of TBCs was discussed. In addition, designs for TBCs to resist spallation were discussed. The main conclusions are as follows: (i) Significant changes occurred for the porosity, 2D pore density and hardness of LZO coatings during thermal exposure. Compared with porosity, the changing degree of 2D pore density was well consistent with the hardness. Therefore, the 2D pores are the essential microstructural characteristics responsible for the changes of mechanical property. (ii) The healing of 2D pores increases the driving force for cracking of LZO coatings. This is the main cause for the spallation of ceramic top coat during thermal cyclic test. The different spallation patterns in iso- and gradient-thermal cyclic tests are mainly attributed to the differential changes in fracture toughness.

Understanding of the dominant effects of 2D pores on mechanical properties and failure of TBCs would make fundamental contribution to the designs of advanced TBCs for future applications.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant number 51901181); the China Postdoctoral Science Foundation (grant number 2018M631151); the Domain Foundation

of

Equipment

Advance

Research

of

13th

Five-year

Plan

(grant

number

JZX7Y20190262062001); the Natural Science Foundation of Shaanxi Province (grant number 2019JQ-165); the Young Talent fund of University Association for Science and Technology in Shaanxi, China (grant number 20190403). The financial support from China Scholarship Council (CSC) to be a postdoctoral researcher in Forschungszentrum Jülich would be greatly appreciated by Dr. G.R. Li (grant number 201806285079).

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Conflict of interests The authors declare no competing financial interests.

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